Since the plasma membrane has been observed to have altered physiological responses and an altered protein composition in neoplastically transformed cells, it is important to further understand the structure of this organelle. Most of the previous studies on plasma membrane proteins have looked for qualitative or quantitative differences between transformed and untransformed cells. Many of the protein species are the same in both transformed and normal cells but there is a strong indication that their spatial organization may be perturbed during transformation. Our studies are designed to elucidate the nature of membrane protein-protein relationships. We have chosen to study whether the transferrin receptor, which is present in increased amounts on a wide variety of neoplastic cells, can be cross-linked to other proteins in/or adjacent to the plasma membrane. We have chosen the transferrin receptor because we have been able to obtain both monoclonal and polyclonal antibodies directed against this receptor. Also, the receptor combined with transferrin can be identified or isolated using antitransferrin antibodies. Using these antibodies, we have been able to show that we can cross-link with dithiobis(succinimidylproprionate) the transferrin receptor into a form which remains with an insoluble matrix after treatment of HeLa cells with Triton X100. We are presently analyzing this matrix for any protein-protein relationships between the transferrin receptor and other proteins from 35S-methionine-labeled cells. Also, we are titrating the effects of dithiobis(succinimidylproprionate) on the solubilization of the transferrin receptor from cells. Using similar techniques, we are beginning to study whether other cell surface receptor molecules can be analyzed in a similar manner. Studies on the effect of receptor-ligand interactions on the physical state of the receptors and its relationships to other proteins are in progress. These studies should provide basic insight into the structural organization and alterations in structure which can occur in cells and a better basis for understanding of cellular physiology.